Bulk metallic glasses (BMGs), which are also known as bulk solidifying amorphous alloy compositions, are a class of amorphous metallic alloy materials that are regarded as prospective materials for a vast range of applications because of their superior properties such as high yield strength, large elastic strain limit, and high corrosion resistance.
A unique property of BMG is that they have a super-cooled liquid region (SCLR), ΔTsc, which is a relative measure of the stability of the viscous liquid regime. The SCLR is defined by the temperature difference between the onset of crystallization, Tx, and the glass transition temperature, Tg of the particular BMG alloy. These values can be conveniently determined by using standard calorimetric techniques such as DSC (Differential Scanning Calorimetry) measurements at 20° C./min.
Generally, a larger ΔTsc is associated with a lower critical cooling rate, though a significant amount of scatter exists at ΔTsc values of more than 40° C. Bulk-solidifying amorphous alloys with a ΔTsc of more than 40° C., and preferably more than 60° C., and still more preferably a ΔTsc of 70° C. and more are very desirable because of the relative ease of forming. In the supercooled liquid region the bulk solidifying alloy behaves like a high viscous fluid. The viscosity for bulk solidifying alloys with a wide supercooled liquid region decreases from 1012 Pa·s at the glass transition temperature to 107 Pa·s and in some cases to 105 Pa·s. Heating the bulk solidifying alloy beyond the crystallization temperature leads to crystallization and immediate loss of the superior properties of the alloy and it can no longer be formed.
Superplastic forming (SPF) of an amorphous metal alloy involves heating it into the SCLR and forming it under an applied pressure. The method is similar to the processing of thermoplastics, where the formability, which is inversely proportional to the viscosity, increases with increasing temperature. In contrast to thermoplastics however, the highly viscous amorphous metal alloy is metastable and eventually crystallizes.
Crystallization of the amorphous metal alloy must be avoided for several reasons. First, it degrades the mechanical properties of the amorphous metal alloy. From a processing standpoint, crystallization limits the processing time for hot-forming operation because the flow in crystalline materials is order of magnitude higher than in the liquid amorphous metal alloy. Crystallization kinetics for various amorphous metal alloys allow processing times between minutes and hours in the described viscosity range. This makes the superplastic forming method a finely tunable process that can be performed at convenient time scales, enabling the net-shaping of complicated geometries. Since similar processing pressures and temperatures are used in the processing of thermoplastics, techniques used for thermoplastics, including compression molding, extrusion, blow molding, and injection molding have also been suggested for processing amorphous metal alloys.
Amorphous metal alloys are an ideal material for small geometries because they are homogeneous and isotropic. This is due to the fact that no “intrinsic” limitation such as the grain size in crystalline materials is present. Also, since thermoplastic forming is done isothermally and the subsequent cooling step can be carried out slowly, thermal stresses can be reduced to a negligible level.
Particularly interesting for small scale applications like MEMS (micro-electro-mechanical-systems), microstructures, NEMS (nano-electro-mechanical-systems), and nanoimprinting, these materials exhibit an isotropic and homogeneous structure even below the length scales of interest. However, these applications are still limited by the difficulties associated with net-shape fabrication.
Two fundamentally different processing routes can be used. The first processing method, direct casting of three-dimensional components, requires simultaneous filling and fast cooling of complex dies, which makes high aspect ratio geometries challenging. Furthermore, direct casting relies on wetting of the mold material by the amorphous metal alloy, which limits for example the use of silicon as a mold material and is not compatible with most of MEMS fabrication methods. An alternate method is thermoplastic forming (TPF), which has been explored for a wide range of processes including net-shape processing, micro and nano replication, extrusion, synthesis of amorphous metallic foams, blow molding, and synthesis of amorphous metal alloy composites. Thermoplastic forming of the amorphous metal alloy is enabled by the existence of a supercooled liquid region (SCLR), as discussed above.
To form amorphous metal alloys using a superplastic forming process, the amorphous metal alloy must be in its amorphous state, which means that the feedstock must be processed so that the sample is cooled fast to avoid crystallization. During this step, the amorphous metal alloy is typically not formed into its final shape but is rather cast into simple geometries such as cylinders, pellets, and powders. Thereafter, the amorphous metal alloy is hot formed by reheating the material into the supercooled liquid temperature region where the material is formed under isothermal conditions, such that the amorphous stage relaxes into a highly viscous metastable liquid that can be formed under applied pressure. Under isothermal conditions, the formability of the amorphous metal alloy increases with increasing processing temperature. Thus, the highest isothermal formability can be achieved at the highest possible processing temperature, so long as crystallization can be avoided.
The ability of an amorphous metal alloy to be thermoplastically formed is described by its formability a parameter which is directly related to the interplay between the temperature dependent viscosity and time for crystallization. Crystallization has to be avoided during TPF of an amorphous metal alloy since it degrades the amorphous metal alloy's properties and retards its formability. Therefore, the total time elapsed during TPF of the amorphous metal alloy must be shorter than the time to crystallization.
Surprisingly the low viscosities that can be reached in the SCLR in some BMGs are sufficiently low that the pressure generated by the surface tension alone is sufficient to deform the BMG. In other words when the BMG is heated into the SCLR under appropriate processing conditions it reduces its surface area, as a cause it gives smooth surface. This for example enables to erase (locally or globally) features such as dots on the surface. In addition, it can be utilized to smooth the surface of final parts. Amorphous metal alloy materials and liquids also show very different temperature dependencies of the viscosity and surface tension. Surface tension typically shows a linear temperature dependence whereas viscosity shows an exponential dependence. Therefore, the ratio of surface tension force/viscous force increases with increasing temperature.
Imprint lithography has become an emerging lithographic technique that promises high throughput patterning of nanostructures on large areas. Various imprint lithography methods have been proposed, as described for example in U.S. Pat. No. 5,772,905 to Chou, the subject matter of each of which is herein incorporated by reference in its entirely. Based on the mechanical embossing principle of a polymer, imprint lithography can achieve pattern resolutions beyond the limitations set by the light diffractions or beam scattering in other conventional techniques.
At the core of the imprint lithography technology is the development of imprint material, which traditionally have included polymer materials with a low glass transition temperature (i.e., a Tg of less than about 105° C.), such as poly(methyl methacrylate) PMMA, given by way of example and not limitation. However, this imposes several major limitations to further improvement of current imprint lithography techniques, including (1) short useful lifetime of the mold; (2) low printing speed; and (3) solubility of the imprint media in organic solutions.
Lifetime of mold: nanoimprint molds presently require replacement after approximately 50 consecutive imprints. Heating and cooling cycles and high pressure (approximately 50-130 bar), applied during embossing, produce stress and wear on the nanoimprint molds.
Printing speed: the low thermal conductivity of polymeric materials (i.e., thermal conductivity of about 0.2 W/m-K) limits the thermal cycling of the substrate to one imprint per 10 minutes.
Solubility of imprint media: the imprint media are soluble or swellable in organic solutions, which prevents further cleaning or repairing of patterned substrates.
Thus, it would be desirable to develop a new imprint technique based on non-polymeric materials. The inventors of the present invention have determined that amorphous metal materials based on bulk metallic glasses (BMG) can be formed into features below about 10 nm. These BMG offer very unique physical properties suitable for nanoprinting. For example, these BMG materials demonstrate thermal conductivity of about 7 W/m-K, which is low for a metal. Typically metals have a thermal conductivity in excess of about 20-450 W/m-K. This low value of thermal conductivity allows for adiabatic heating of (intimate vicinity of patterned feature) in a serial imprinting process. In addition, as compared to the thermal conductivity of polymeric materials, which generally runs at about 0.2 W/m-K, the thermal conductivity for amorphous metals is significantly larger, which is highly beneficial for the reduction in the thermal cycle time in a parallel imprinting process.